A mining machine of the type in which a plow trough is disposed ahead of a conveyor trough, one or more plows are carried by an endless chain passing around reversing stations at opposite ends of the trough and the chain also has conveyor flights which are displaceable in a conveyor trough rearwardly of the plow trough to displace the mined material, is known inter alia, from German patent document DE 43 19 512 A1.
At each of the reversing stations, a sprocket wheel is provided and the endless, statically tensioned chain, passes around these sprocket wheels.
One of the sprocket wheels, i.e. the one at the end of the conveyor pass and at the upstream side of the plow pass, is connected to a conveyor drive while the other sprocket wheel in the region of the upstream side of the conveyor pass and the downstream end of the plow pass is connected to a plow drive. Both drives are operated to displace the chain and each drive can be a three-phase alternating current motor connected to the shaft of the respective sprocket wheel.
Furthermore, at least downstream of the sprocket wheel of the conveyor drive, insertion or guide elements are provided for feeding the plow bodies and the conveyor bodies into the respective guides of the plow trough.
In the continuous operation of such machines, chain sag can develop. The chain sag can result from a clamping of the chain along the plow pass or an increase in resistance in the mining operation.
Because of the high tension on the chain in the plow pass, the chain can stretch somewhat to form the sag. Since the chain tension is somewhat less in the conveyor pass, the chain usually is free from stretch in this position. Where the deformation is greater than the static tension on the chain, the actual sag formation develops. The sag is effectively an increase in the length of the chain and is similar to the sag which arises in cables or the like.
The formation of said sag can be avoided by providing the chain under extremely high pretension. While this is effective for the mining of uniform seams or even seams in which changes in the seam structure or the mineral structure is slight, problems are encountered when the seam contains rock or regions of varying hardness and considerable force is required to overcome engagement of the plow with these parts of the seam to be mined.
Even though a greater force may be applied, sag may nevertheless develop. Since the sag is particularly problematical immediately downstream of the conveyor drive sprocket wheel, where the aforementioned guide elements are provided, the plows and/or the conveyor flights may not be properly guided into the plow trough and can cant, twist or slip out of place, requiring time-consuming and expensive operations to repair the apparatus.